This application claims priority of European Patent Application No. 00302984.0, which was filed on Apr. 7, 2000.
Related subject matter is disclosed in the following application assigned to the same assignee hereof: U.S. Patent application entitled xe2x80x9cRF Resonator,xe2x80x9d Ser. No. 09/825,119, filed Apr. 3, 2001, now abandoned.
1. Field of the Invention
The present invention relates to a resonator for use at radio frequency (rf), especially microwave frequencies, for use in telecommunications systems.
2. Description of the Related Art
Resonators are commonly used at microwave frequencies in filters, etc. since circuits formed of separate inductors and capacitors cannot easily be fabricated for use at microwave frequencies. Microwave resonators may take a variety of forms, but a common type is a short section of transmission line, a quarter wavelength or half a wavelength long and appropriately terminated. The transmission line may, for example comprise coaxial cable, microstrip, in which a strip conductor is separated from a metal groundplane by a layer of dielectric, or strip line in which a central strip conductor is separated from two opposing groundplane conductors by two layers of dielectric on either side of the strip conductor.
The properties of transmission lines employing superconductive films as conductive plates have been studied. Superconducting films commonly employ high temperature superconductors (HTS) such as YBCO. Commonly, these have a critical temperature in a range which terminates above 1000 K. In practice such films operate with cryogenic systems employing liquid nitrogen and operating at a temperature of 770 K, the boiling point of liquid nitrogen. The properties of superconducting films in transmission lines, and the temperature dependence of the constituents of such transmission lines, are discussed in the following references:
[1] F Abbas, and L E Davis, xe2x80x9cPropagation coefficient in a superconducting asymmetric parallel-plate transmission line with buffer layerxe2x80x9d, J. Appi. Phys. 73, pp. 4494-4499, 1993.
[2] X D Wu, A barn, M S Hegde, B Witkens, C C Chang, D M Hwang, L Nazar, T Venkatersan, S Miura, S Matsubara, Y Miyasaka, Appl. Phys. Lett. 54, 754 (1989).
[3] S Y Lee and H H Park, J. Superconductivity, 9, 545 (1996).
[4] N F Mott, Advances Phys. 39, 55 (1990).
[5] C Gallop, C D Langham, L Hao and Farhat Abbas, IEEE Trans. Instrument, and Measurement. 46, (1997).
The invention is based on the recognition that a resonator employing superconducting films may be constructed with an extremely stable resonant frequency value for changes in temperature. Further if, as is possible with cryogenic systems, the temperature is controlled very accurately, the resonator may exhibit zero, or very close to zero change in its operating parameters over the range of the controlled temperature. In particular, it has been found for a small change in temperature, 1 mKxc2x0, that the present invention can provide a resonant frequency stable to within 1 part in 1015.
It has been further recognized that the present invention is applicable more generally to resonators which employ normally conductive layers. Further, it has been recognized that it is not necessary to employ conductive layers at all to achieve the beneficial effects of the invention.
The present invention provides an electromagnetic resonator comprising: a dielectric substrate of predetermined material having a predetermined width and thickness, and having a predetermined length in the direction of propagation for achieving a desired resonance; first and second temperature compensating dielectric layers on two opposite faces of the substrate and extending along the length of the substrate, the dielectric layers being of a predetermined material and having a predetermined thickness; and the arrangement being such that the wave velocity of the resonator is dependent on the parameters of the substrate and first and second temperature compensating layers whereby the temperature dependence of the frequency of resonance of the resonator can be maintained within a predetermined range over a predetermined temperature range.
The temperature dependence of the temperature compensating layers can be of opposite sign to that of the substrate.
The first and second conductive layers can be formed on the outer surfaces of the respective first and second temperature compensating dielectric layers. The conductive layers can be a normal conductor such as copper or, as preferred, HTS superconducting layers such as YBCO. As will be shown below, it is desirable to select the parameters of the temperature compensating layers such that at a selected temperature, the first derivative with respect to temperature of the phase velocity of the electromagnetic wave propagating in the resonator is zero at the operating temperature of the resonator.
In accordance with a preferred form of the invention, an expression for the wave velocity is provided, the first and second temperature derivatives of this wave velocity with respect to temperature are made zero or at any rate to a non-significant value by appropriate choice of materials and layer thicknesses in accordance with the wave velocity expression. Thus, the resonant frequency of a superconducting planar resonator is dependent on the material properties and thicknesses of the superconductors, the dielectric substrate and the temperature compensating layers. As preferred, the first and second derivatives with respect to temperature of a wave velocity ratio (with respect to free space) are put to zero for various combinations of material properties. For YBCO thin films on rutile with sapphire temperature compensating layers a turning point can be realised at T=60xc2x0 K. As the temperature in cryogenic systems can be controlled to better than 0.1xc2x0 mK, then frequency standards with stabilities of parts in 1015 are attainable.
In a further aspect, the present invention provides a procedure for stabilizing the resonant frequency of an electromagnetic resonator with respect to temperature comprising providing a dielectric substrate of predetermined width and thickness, of predetermined length in the direction of propagation for achieving a desired resonance, and having a dielectric constant; providing first and second dielectric layers on two opposite faces of the substrate and extending along the length of the substrate, each layer having a thickness and a dielectric constant; providing first and second conducting layers on the outer surfaces of the first and second dielectric layers having a thickness and a penetration depth for electromagnetic fields; and selecting materials, thicknesses and dielectric constants of one or more of the aforesaid layers in relation to the thickness and dielectric constant of the substrate so as to achieve a desired stability in resonant frequency over a range of temperatures.